PLACING CONCRETE Transporting Concrete.—The methods of handling concrete from the mixer to its final location vary with the size of the work and the consistency of the concrete.
For small work and short distances, wheelbarrows are commonly used. Ordinary contractors' barrows carry from about 1.S to 2.0 cubic feet at a load. For longer hauls, two-wheeled barrows, carry ing about 6 cubic feet, are more economical. On large work, cars running on temporary tracks are frequently employed, or when the work is within a short radius, derricks may be used. In building operations concrete is frequently raised by a bucket hoist to the required elevation and distributed by barrows to the various parts of the work.
These methods of handling, in which a small bulk of the concrete is held together in transportation and dumped at once into place, offer little opportunity for the ingredients of the concrete to separate. Well-mixed concrete of any proper consistency may be transported to considerable distances without being injured. Concrete that is so dry as to lack cohesion, or concrete that is so wet that the mor tar is soft enough to run away from the stone, shows a tendency to separation in handling, and these consistencies should not be used.
Transportation in distribution of concrete is some times effected by elevating it sufficiently to permit it to flow in a trough or chute to its destination, and by arranging a system of movable chutes it is often possible to distribute over considerable area from a single hoisting tower. When the mixer can be set above the work, as in foundations or sometimes in dams and similar struc tures, the concrete may be transported wholly by gravity.
To flow in chutes, rather soft, mushy concrete is necessary, unless the chutes are quite steep. When the slope of the chute is very flat, the concrete roust be made very wet, and does not result in first-class work, while the extra water necessary to make the concrete flow on a flat slope causes the mortar to separate from the stone, arid frequently washes portions of the cement from the mortar. The practice of adding water in the chutes to assist the
flow is always detrimental.
Experience indicates that concrete may be made to flow readily in chutes on slopes from about 20° to 35° to the horizontal; for any slope less than about 20°, the concrete must be made too wet. The mass of concrete should slide along the chute as a whole, the stone and mortar traveling together at common velocity. For ordinary mushy concrete, as commonly used in reinforced work, a slope of 2 horizontal to 1 vertical is found most efficient.
Pneumatic Transportation., by forcing the concrete through pipes by compressed air, has been used in seine instances—a method avail able on congested work, where space is lacking for other means of transport, as in tunnel and subway 83. Depositing Concrete.—When concrete is mixed dry (the consistency of damp earth) and placed in mass construction, it is usually placed in layers about 6 inches deep and each layer tamped until the mortar flushes to the surface. Concrete so mixed and placed attains greater strength than if mixed with more water. If dry concrete is not tamped so as to be thoroughly compacted, it is more porous and has less strength than wet concrete; the labor required in properly placing dry concrete is considerable and the work strenu ous, so that for ordinary uses dry concrete is not commonly employed. Poor work has frequently resulted from the use of dry concrete not properly compacted.
In ordinary practice concrete is mixed either to a rather stiff plastic condition or to a softer mushy consistency. Plastic con crete, when in massive work, should he spread in layers not more than 10 or 12 inches deep and lightly rammed; the mortar should readily flush to the surface and the mass quake like jelly under the ramming. The rammer is usually a flat piece of iron about 6 inches square, with a vertical handle, and weighing 15 to 20 pounds. Smaller tapering ranuners are also used for compacting next to the forms.